Construction machines such as an excavator and a skid steer loader includes a plurality of joint portion, for example, the connecting portions between a boom and an arm, the connecting portions between an arm and a bucket. In the joint portion, a pin and a bush generally oscillate under lubrication condition by grease.
FIG. 2 illustrates a front joint portion 10 of an excavator, as an example of construction machines. FIG. 5 is a cross-sectional view of a bush of the joint portion 10, taken perpendicular to the longitudinal direction of a pin. The joint portion 10, as illustrated in FIG. 5, is composed of a pin 12 connecting two members requiring a joint portion motion and a bush 11 reducing friction resistance between the pin and a pin hole. The pin 12 and the bush 11 generally oscillated under lubrication condition by grease. Further, the joint portion 10 includes, other than the pin 12 and the bush 11, a boss 13 of a housing supporting the pin and the bush, and a sealing (not illustrated) for blocking dirt.
The joint portion operates under various conditions in accordance with the connection type between the pin and the bush, the operational environment, and the type of the joint portion. They generally operate at surface pressure of about 2 to 6 kg/mm2 and a speed of about 0.25 to 5 cm/sec. The values indicated above are average one. Surface pressure may rapidly increase, depending on the work load, and instantaneous surface pressure may be three times or more of an average value due to eccentric load in working, depending on the operation condition.
The bush used in high surface pressure and a low speed requires high resistance against scuffing and wear resistance. So, products of the bush with various composites and types have been developed and used.
For example, these are a metal bush of which the lubrication ability is maximized by forming grooves or dimples on the inner circumferential surface of a metallic rigid body and putting grease in the grooves or the dimples; a metal bush of which the lubrication ability is maximized by coating its surface with a self-lubricative material; and a porous-sintered metal bush in which a lubricant.
However, in the front sections of construction machines, the direction and level of load applied to each parts are difficult to be specified and also frequently changed in accordance with the configuration of the ground, the position of the machines, the geological features. Therefore, in working, each parts may be bent or eccentric load may arise, and accordingly, high load concentrates on local regions. Thus, all the parts including a bush require high-level wear resistance and shock resistance.
However, metal bushes that have been known up to now have a limited elastic strain regardless of shape thereof, so high load to local regions cannot be avoid under the condition that the direction and level of load are difficult to be specified and are changeable, as described above. Further, even if the surface of metal bushes is coated with a self-lubricative material, the lifespan of the self-lubricative material is limited, so it is difficult to keep the lubrication environment after initial stabilization step of the bush surfaces. On the other hand, when a porous metal-sintered body is continuously exposed to shock, pores in the sintered body gradually decrease from the surface where load is applied. Further, if severe, the pores are closed, so the lubricating effect by the lubricant impregnated in the sintered body may not be maintained.